End-burning propellant grain with area-enhanced burning surface

ABSTRACT

An end-burning grain of a solid rocket motor or other gas-generating device is supplemented with one or more sticks of high-burn-rate propellant embedded in a matrix of a relatively low-burn-rate propellant. The sticks increase the burning surface area as the grain burns by forming conical indentations in the surface.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional PatentApplication No. 61/354,856, filed Jun. 15, 2010, the contents of whichare incorporated herein by reference.

BACKGROUND OF THE INVENTION

End-burning grains are used in solid propellant rocket motors andpyrotechnic gas generators that require stable and prolonged generationof combustion gases. In addition to these qualities, end-burning grainsreduce the chances of spalling of the propellant and of unintentionalignition upon impact from bullets or fragments. An end-burning grainalso allows a motor to accommodate a high volume of propellant, and whendesired, the propellant can be loaded in a cartridge for easy placementin the motor case. Examples of end-burning rocket motors are theoriginal Stinger (a surface-to-air missile developed by the UnitedStates Army), the ARCAS (All-Purpose Rocket for Collecting AtmosphericSoundings, first developed by Atlantic Research Corporation), and theStandard Missile Sustainer motors (also developed by Atlantic ResearchCorporation).

In an end-burning motor, the length of the propellant grain establishesthe duration of the thrust, while the magnitude of the thrust depends onthe mass burning rate which is determined by the choice of propellant aswell as the area of the burning surface. The mass burning rates of solidpropellants are limited, however, and while propellant compositions canbe modified to achieve higher burn rates, these modifications tend toproduce smoke, toxic gases, or both. The smoke emitted fromshoulder-launched rockets such as the Stinger obscures the user's visionand risks eye damage, and smoke emitted from tactical rockets launchedfrom aircraft or ground vehicles impairs the visibility of the pilot ordriver. Smoke also leaves a visible trail that can indicate the sourceof the launched rocket.

To address these problems, reduced-smoke and minimum-smoke propellantshave been developed. Unfortunately, the ballistic properties of suchpropellants, particularly the minimum-smoke propellants, are generallyundesirable because their burning rates are temperature-dependent, asopposed to “plateau” or “mesa” burning rates, i.e., burning rates thatare insensitive to temperature over a wide range of operating pressures.Propellants with “plateau” or “mesa” burning rates are capable ofproviding substantially constant thrust regardless of firing conditionsand operating temperature. Burning rate modifiers that are specificallydesigned to achieve “plateau” or “mesa” burning rate behavior are oftenincluded in minimum-smoke propellants, and the modifiers of choice forthis purpose are lead salts and lead-containing compounds, as describedin U.S. Pat. No. 3,138,499 (A.T. Camp et al., inventors, issue date Jun.23, 1964). Lead salts and related compounds present problems, however.They are toxic, they complicate the propellant manufacturing process,they produce exhaust products that are hazardous to personnel,particularly during training exercises involving the use of the rockets,and they are harmful to the environment in which the training exercisesare conducted. For these reasons, training in the use of rockets withminimum-smoke propellants must include expensive range remediation fromthe toxic by-products.

Enlargement of the area of the burning surface to increase thrust can beachieved by increasing the motor diameter, but this too has itslimitations, since there are practical limitations on how large themotor diameter can be.

SUMMARY OF THE INVENTION

The present invention resides in an end-burning rocket motor with apropellant grain that is capable of serving as a minimum-smokepropellant and also has a modified exponent, i.e., a lowered sensitivityto pressure changes at high pressures. The propellant grain of thisinvention produces localized concave regions in the burning surfaceduring the course of burning, thereby increasing the burning surfacearea relative to that formed during unassisted or uniform burning acrossthe burning surface. The grain of the present invention maintains andexpands the increased area as the propellant bums and does so withoutthe need for additives, especially lead salts and lead-containingcompounds, that increase the amount of toxic by-products produced by themotor.

The increased surface area is achieved by the use of a matrix propellantthat has one or more faster-burning sticks, i.e., rods of small crosssection relative to the grain as a whole, embedded in the matrixpropellant, the sticks being of a propellant with a burn rate that ishigher than the burn rate of the matrix propellant. The stick(s) extendfrom the starting burn surface into the bulk of the matrix propellant,and in many cases they extend the full length of the matrix propellant.Upon ignition of the grain from the ignition (aft) end, the relativelyhigh heat generated by the faster-burning stick(s) causes the regions ofthe matrix propellant that are closest to each stick to burn faster thanregions that are further away. The matrix propellant thus burns at ratesapproximately proportional to the radial distance from each stick,producing localized concave regions that are at least approximatelyshaped as right-circular inverse cones. In certain embodiments of theinvention, the propellant grain is shaped to include small cone-shapeddepressions or other surface features in the burn surface prior toignition. These surface features help to initiate the growth of theconcave regions during burning or to otherwise control the initialrocket motor performance. In general, however, the ballistics of themotor in terms of burn rate behavior are controlled by thehigh-burning-rate stick(s) while the matrix propellant determines therocket motor performance in terms of the operating pressure andcombustion characteristics of the propellant. The use of the stick(s)allows a wide latitude in matrix materials, and also allows metallicadditives such as lead and smoke-generating materials such as ammoniumperchlorate to be eliminated without significant loss of performance andwithout significant increase in smoke generation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective cross section view of a propellant grain withinthe scope of the present invention.

FIG. 2 is a longitudinal cross section of the grain of FIG. 1, showingthe burn profile.

DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS

The material of which the sticks are formed, hereinafter referred to as“ballistic control propellant,” and the matrix propellant are solidrocket propellants, including many of the propellants known in the art.The ballistic control propellant and the matrix propellant aredistinguishable from each other by their burn rates. Although thespecific materials used in any single embodiment of the invention canvary widely, best results will in most cases be obtained when the burnrate of the ballistic control propellant is from about 1.3 times toabout ten times the burn rate of the matrix propellant, or from about1.5 times to about five times, or from about 1.5 times to about threetimes. Expressed as the burning rates themselves, ballistic controlpropellants can have, for example, burning rates within the range ofabout 0.75 in/sec (1.9 cm/sec) to about 5.0 in/sec (12.7 cm/sec) at2,000 psi, or from about 0.85 in/sec (2.2 cm/sec) to about 3.0 in/sec(7.6 cm/sec) at 2,000 psi, and matrix propellants can have burning rateswithin the range of about 0.25 in/sec (0.64 cm/sec) to about 2.0 in/sec(5.1 cm/sec) at 2,000 psi, or from about 0.4 in/sec (1.0 cm/sec) toabout 1.0 in/sec (2.5 cm/sec) at 2,000 psi, with the difference betweenthe burning rates of the two propellants being within the range of about0.25 in/sec (0.64 cm/sec) to about 2.0 in/sec (5.1 cm/sec), andpreferably from about 0.4 in/sec (1.0 cm/sec) to about 1.0 in/sec (2.5cm/sec). In a presently contemplated example, the ballistic controlpropellant has a burn rate of 1.0 in/sec (2.5 cm/sec) at 2,000 psi, andthe matrix propellant has a burn rate of 0.55 in/sec (1.4 cm/sec) at2,000 psi.

The ballistic control propellant can be either a homogeneous propellantor a composite propellant, and the same is true for the matrixpropellant. Composite propellants can include fuels, oxidizers, andbinders, particularly energetic oxidizers and binders. Indeed, the term“propellant” as used herein is intended to include energetic polymers,energetic oxidizers, and binders, and propellant compositions canfurther include additional components such as stabilizers and modifiers.The ballistic control propellant can include nitrocellulose, forexample, in combination with an energetic plasticizer such as, forexample, cyclotetramethylene tetranitramine (HMX), cyclotrimethylenetrinitramine (RDX), butyl nitratoethyl nitramine (BuNENA), butanetrioltrinitrate (BTTN), bis-dinitropropylacetal/formal (BDNPA/F), ormethyl/ethyl nitratoethyl nitramine (Me/Et NENA), and further incombination with a high- nitrogen burning rate modifier such as, forexample, triaminoguanidinium azotetrazolate (TAGzT), guanidiniumazotetrazolate (GUzT), 1,1-diamino-2,2-nitroethene (FOX-7),3,3′-diamino-4,4′-azoxyfurazan (DAAF), or2,6-diamino-3,5-dinitropyrazine-1-oxide (LLM-105). The ballistic controlpropellant preferably has a burn rate that is greater than 1 inch persecond at 3,000 psi, and is thermally insensitive. For the matrixpropellant, examples of energetic components of particular interest arecyclotetramethylene tetranitramine (HMX), cyclotrimethylene trinitramine(RDX), bis-nitrofurazanyl furoxan (BNFF), 3,3′-dinitro-4,4′-furazanyloxamide (DNFOA), methylene-bis-aminonitrofurazan (MBANF),2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20), andammonium 2-oxy-5-nitrotetrazolate (AONT). In the ballistic controlpropellant, and in certain cases, both the ballistic control propellantand the matrix propellant, preferred formulations have a low pressuresensitivity at high pressures. This sensitivity is expressed in the artas the exponent, which is a measure of the increase in burning rate of apropellant which occurs as the chamber pressure is increased. Preferredformulations are those in which the exponent exhibits “plateau” or“mesa” burning rate behavior at operating pressures, and in many casesthose in which the exponent is less than 0.5 at 1,000 psi.

The thicknesses of the ballistic control propellant sticks, the numberof the sticks, their spacing from each other and from the walls of themotor casing, and their placement (i.e., relative positions) within thematrix propellant can all vary and are not critical to the invention. Inmany, but not all, embodiments, the number of sticks is one to thirty;in certain of these, the number is three to twenty; and in certain ofthese, the number is five to twenty. The diameter of a stick inpreferred embodiments is within the range of about 0.1 inch (0.25 cm) toabout 1.0 inch (2.54 cm), and in many cases from about 0.1 inch (0.25cm) to about 2.0 inch (0.51 cm). Expressed in terms of the ratio of thetotal cross section area of the sticks to the total cross section areaof the propellant grain, including both the sticks and the matrixpropellant, this ratio can be from about 1×10⁻⁴ to about 3×10⁻³, andwill often be from about 3×10⁻⁴ to about 1×10⁻³. In one presentlycontemplated embodiment, the propellant grain is approximately 5.5inches (14 cm) in diameter and 10.5 inches (26.7 cm) in length withseven sticks of embedded ballistic control propellant, eachapproximately 0.14 inch (0.36 cm) in diameter and 10.5 inches (26.7 cm)in length, one stick at the center of the grain and the remaining sixdistributed in a circle around the center, approximately two-thirds tothree-fourths of the distance from the center to the casing wall.

The sticks of ballistic control propellant can be aligned parallel tothe longitudinal axis of the motor casing or to the casing wall, and formotors that are right circular cylinders with a central axis, the stickswill often be parallel to the central axis. When a single stick is used,the stick is conveniently positioned along (coincident with) the centralaxis. When two or more sticks are used, the sticks can be arrangedsymmetrically around the axis, either with a single stick along theaxis, or all sticks equidistant from the axis. An alternative toalignment of the sticks parallel to the axis is an orientation in whichthe sticks are angled relative to the axis but have a component parallelto the axis. This includes sticks that are angled outward from the axisin either direction relative to the burn direction, as well as sticksthat are spiral in shape.

The propellant grain, including the sticks of ballistic controlpropellant and the matrix propellant, can either be formed in the motorcasing directly, or pre-formed as a single coherent solid mass orcartridge and inserted in the casing, all such methods being known inthe rocketry art. The sticks will in many cases be formed first andpositioned according to their final position in the grain, and thematrix propellant will then be formed or placed around the sticks. Whenformed in the motor casing or in a cartridge casing, the matrixpropellant can be cast from a liquid solution or slurry containing aheat-curable binder and heat cured in place. Other methods of forming apropellant grain in an end-burning motor will be readily apparent tothose of skill in the art. The grain can be securely bonded to thecasing in which it is cast, without gaps, to avoid localized gas pocketsor gas flow in directions other than the intended direction, i.e., theaft direction from the end surface. Bonding at the casing wall islikewise accomplished by conventional means, such as for example the useof the same binder that binds the grain into a coherent mass.

The matrix propellant can be devoid of metallic fuels, examples of whichare aluminum and boron, and also devoid of halogen-containing additives,including chlorine-containing oxidizers, of which a prominent example isammonium perchlorate. The matrix propellant can likewise be devoid ofmetallic ballistic modifiers, such as lead, lead compounds, or leadsalts. When lead in any form is present, the use of a ballistic controlpropellant in the fond of relatively thin sticks allows a reduction inthe lead by confining the lead to the sticks. In certain embodiments ofthe invention, however, no lead is included in any part of thepropellant grain, including the sticks. Both the ballistic controlpropellant and the matrix propellant can also be devoid of RDX.

FIGS. 1 and 2 depict one example of a propellant grain configuration inaccordance with this invention, and the burn profile of the grain.

FIG. 1 is a transverse cross section of the grain in perspective,showing the grain contained in a boot 11 which is in turn surrounded bya heavywall case 12, both of which are essentially bodies of revolutionabout a central longitudinal axis 13. The grain includes seven sticks 14of ballistic control propellant, with one stick along the longitudinalaxis 13 and the remaining six arranged in a circle centered around theaxis 13, all sticks parallel to the axis 13 and extending the fulllength of the grain. The remainder of the grain is the matrix propellant15.

FIG. 2 is a longitudinal cross section of the motor, taken along a planeparallel to the longitudinal axis but offset from the axis to intersectonly one of the sticks 14, and specifically a stock that is other thanthe stick at the axis of the motor. The direction of burn is indicatedby the arrow 21, and the initial burn surface 22, i.e., the surface atwhich ignition occurs, is at the aft end of the grain. The transverselines 23 in the grain represent the burn profile at increasing times,from initiation (t=0) at the far right (where the burn surface is flat)to progressive times toward the left. As shown in the profile, acone-shaped indentation 24 forms in the burn surface in the early stagesof burning, and the diameter of the indentation increases as the burnprogresses to the point where the burn surface lacks a flat portionentirely. The slanted lines of the burn profile at the opposite side ofthe grain are a portion of the growing cone of another stick.

The foregoing description is offered primarily for purposes ofillustration. Further modifications, variations, examples, andsubstitutions that will be apparent to those skilled in the art arelikewise included in the scope of the invention.

In the claims appended hereto or any claims subsequently added, the term“a” or “an” is intended to mean “one or more.” The term “comprise” andvariations thereof such as “comprises” and “comprising,” when precedingthe recitation of a step or an element, are intended to mean that theaddition of further steps or elements is optional and not excluded. Allpatents, patent applications, and other published reference materialscited in this specification are hereby incorporated herein by referencein their entirety. Any discrepancy between any reference material citedherein or any prior art in general and an explicit teaching of thisspecification is intended to be resolved in favor of the teaching inthis specification. This includes any discrepancy between anart-understood definition of a word or phrase and a definitionexplicitly provided in this specification of the same word or phrase.

1-17. (canceled)
 18. An end-burning grain of a solid gas-generating composition, said grain having an ignition end and a longitudinal axis, said grain comprising: a matrix propellant having a matrix propellant burn rate, wherein said matrix propellant is devoid of metallic fuels and halogen-containing additives, and a rod of ballistic control propellant embedded in said matrix propellant, wherein said ballistic control propellant comprises nitrocellulose, an energetic plasticizer, and a high-nitrogen burning rate modifier and is devoid of ammonium perchlorate, said ballistic control propellant having a ballistic control propellant burn rate that is substantially greater than said matrix propellant burn rate, said rod terminating at said ignition end of said grain, wherein the matrix propellant and the ballistic control propellants are solid propellants.
 19. The end-burning grain of claim 1 wherein said rod is substantially parallel to said longitudinal axis. 